The invention relates to a high-pressure discharge lamp comprising a discharge vessel, which contains an ionizable gas filling, consists of translucent material and is mounted within a translucent outer envelope, which is adjoined at an end thereof by a lamp base, a heat-insulating porous translucent member being arranged between the discharge vessel and the outer envelope. Such a lamp is known from GB PS 481320.
In high-pressure discharge lamps, for example in sodium, mercury or metal halide high-pressure discharge lamps, there is, especially with the vertical operating position in which the lamp base points upwards, the problem of overheating of the lamp base and of parts that may be contained therein. The lamp base may consist either only of the actual lamp cap or of the cap and an additional hollow body arranged between this cap and the outer envelope.
In newer high-pressure discharge lamps, it is known to accommodate an electronic ballast or ignition device in the lamp base (DE-OS 29 39 632, which corresponds to U.S. Pat. No. 4,316,124). Although in this case the lamp cap is covered by a heat-trapping plate, it is in immediate thermal contact with the outer envelope. As a result, the lamp cap can be heated to more than 150.degree. C., which would lead to a destruction of the electronic elements located in the lamp cap. Moreover, in this case there is also need for a particular heat-resistant fitting.
U.S. Pat. No. 2,103,028 discloses a high-pressure discharge lamp, a cylindrical hollow glass member is arranged between the envelope and the cap, which member is to impede the heat transport to the cap because of its small wall thickness and great length. Such intermediate members make the lamp very long and mechanically unstable.
DE-OS 3416714, which corresponds to U.S. Pat. No. 4,644,216, discloses in high-pressure discharge lamp, in which the intermediate member of glass is folded as bellows, which requires, however, a complicated glass processing treatment.
However, the lamp base is heated not only by heat conduction from the outer envelope, but also by heat radiation, which is emitted by the discharge vessel and is absorbed by portions of the lamp base.
GB-PS 481320 discloses a gas discharge lamp, in which an insulating translucent material is present in the space between the discharge vessel and the outer envelope, which material may constitute, for example, a porous envelope of glass wool. Such a material leads to a strong scattering of the light originating from the discharge vessel, as a result of which the focusing of this light becomes more difficult. Therefore, such a material may have only a comparatively loose packing, as a result of which the trapping of heat is limited, however.
This also applies to a lamp arrangement known from EP 0 165 701 A1, in which a halogen incandescent lamp is arranged in an outer envelope. A heat shield and a filling consisting of insulating glass wool are provided between the incandescent lamp and the lamp base.